Fix broken links

Exercises/ex3_8.md

@@ -10,7 +10,7 @@
 
 ## (a) The Trouble with Column Formatting
 
-If you go all the way back to link:ex3_1.txt[Exercise 3.1], you 
+If you go all the way back to [Exercise 3.1](ex3_1.md), you 
 wrote a function `print_portfolio()` that produced a table like this:
 
 ```python
@@ -204,7 +204,7 @@ different ways.
 Using mixins can be a useful tool for framework builders for reducing 
 the amount of code that needs to be written.  However, forcing users
 to remember how to properly compose classes and use multiple inheritance can
-fry their brains.  In link:ex3_5.html[Exercise 3.5], you wrote a 
+fry their brains.  In [Exercise 3.5](ex3_5.md), you wrote a 
 function `create_formatter()` that made it easier to create a custom
 formatter.  Take that function and extend it to understand a few optional
 arguments related to the mixin classes. For example: